Coating and coating method



Patented May 23, 1944 UNITED STATES" PATENT OFFICE OOATING 3532M mi'rnon Joseph L. Krieger, Baltimore, Md.

N Drawing. Application May 19, 1941. Serial No. 394,163

3 Claims. (01. 106-181) 'I'hepresent invention relates to the coating of metals and methods for stabilizing lacquer compositions against discoloration during use. A further object of the present invention is to restore the original color of lacquer compositions which have become discolored during their utilization.

Lacquers are generally composed of nitrocellulose, cellulose acetate or cellulose ether bases in a suitable solvent or mixture of solvents together with diluents or non-solvents, one or more plasticizers and suitable resins. Pigments, carbon black, metal powders, etc., may be added to give any desired colors. Water white or nearly water white lacquers are required where it is desired-to cover highly polished metal surfaces with a thin invisible film to prevent tarnishing of the metal.

Lacquer is generally applied to the article that is to be coated, either by brushing, spraying, dipping and occasionally by pouring The lacquering technic used is suited to the particular conditions, Dip lacquering is the simplest and requires, on the whole, little practice in execution. Dip

lacquering is very widely used. Articles of brass, bronze, Brittania metal, electroplated copper and silver, gold and silver plate, oxidized plastioized wear; in fact, all articles made of the useful metals or of the more valuable alloys are generally covered with a protective coat by dipping in colorless lacquer. A large variety of articles of many sizes and shapes are treated in this manner, including cigarette cases, lighters, compacts, knife handles, childrens toys, ornaments, cheap jewelry, etc. By this treatment, all of these articles are covered by an invisible protective coat.

Nitrocellulose solutions for dip lacquering must have a high solid content and'yet be highly mobile. They must contain sufficient high boiling solvent to insure good flow of the coating.

Dip lacquering is either carried out by hand or by mechanical means. It is important that any excess lacquer be drained off the article coated so as to obtain a uniform film throughout without any blistering. The excess lacquer is run back into its container and used over and over again until it is consumed. As the lacquer becomes viscous thru use and evaporation of solvent, suitable solvents are added from time to time torestore its proper viscosity. In general, articles are pretreated before lacquering to remove dirt, grease and other contaminants.

In the case of the clipping of copper containing articles, it has been found that the colorless lacquers gradually acquire a greenish color which is intensified with further use. This is objectionable particularly in the or $9 of the more expensive articles since the protective film has a greenish hue. This is aggravated at spots where there is any tendency for lacquer to accumulate.

It has now been found that the addition of soluble organic acids to the regular lacquer compositions, said acids having a dissociation constant for the first hydrogen greater than 1X10- is effective in restoring the original color of used lacquers that have turned green thru dipping operations. Moreover, it has also been found that the addition of such acids to the original lacquer in suitable amounts prevents its discoloration when used for dipping.

Among the acids that have beenfound effective for this purpose are monobasic acids, dibasic acids, tribasic acids, hydroxy acids and nitrogen containing acids. Some of these acids, together with their dissociation constants, are as follows:

Dissociation constant for 1st hydrogen Formic acid 2.145(107 Maleic acid 1.5 X10- Citric acid 8 X10- Oxalic acid 3.8 X10 Lactic acid 1.38X10- Dichloracetic acid 5 10-- Picric acid 1.6 x10"- Glycolic acid 2.7 X10- in amounts from 0.1 to 1% or more to preventthe lacquer from turning green upon continuous use.

Although the addition agents described are satisfactory for-the purpose, it has been found that those compounds which are solid at room tem-- perature and Preferably white in color are the most effective. This is due to the fact that the liquid addition agents may prevent Proper adhesion of the lacquer film after evaporation of the solvent or may cause blushing or minute bubbles. Moreover, the use of any colored organic acids will tend to discolor the lacquer and henc should-be avoided. Particularly valuable organic acids for the purpose of this invention are maleic' acid, citric acid, oxalic acid and glycolic acid.

i a greenish C0101.

Example I It was also found that, by adding from 0.1 to 0.2% by weight of citric acid to the discolored lacquer that originally contained citric acid, and,

restored and each of these lacquers could be used v ored lacquer. The original color of the lacquer ,was quickly restored. The lacquer was then used again on similar cigarette cases under "the same conditions as above and acquired a green color after a period of use corresponding to that of the original lacquer. A further addition of 0.1% by weight of maleic acid in the form of a 10% solution in dioxane was made and the color was again restored to what it was originally.

The same procedure was used several times with similar success until finally the lacquer was consumed.

Y Example II To batches of the same nitrocellulose lacquers used in Example I were added 0.3% by weight respectively of citric acid, glycolic acid, pxalic acid and maleic acid and the lacquers used in a mechanical dipping operation on brass and bronze articles and a. batch of the same nitrocellulose lacquer without any of these addition agents was used for a control in the same type of dipping operation.

It was found that the number of articles that similarly, glycolic acid, oxalic acid and maleic acid in 0.1-0.2% by weight to the discolored-lacquers that originally contained these addition agents, the original color of these lacquers was again entirely satisfactorily until they again became discolored. By the periodic addition of these organic acids, it was possible to consume these lacquers in dipping operations successfully.

It is believed that the acidswithin the scope of this invention function by combining with the soluble copper or other metal compounds that are .responsible for the green color to give colorless compouhds'.' However, this invention is not to be limited by any-theory or mechanism of reaction. Obviously many modifications and varia tions of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. A composition of matter consisting of a clear lacquer and 0.1 to 1.0% by weight of maleic acid.

'2. A process for the prevention of the discoloration of clear lacquers used for dipping copper and copper containing materials by the addition thereto of 0.1 to 1.0% by weight of maleic acid.

3. A process for the restoration of the original clear color of lacquers that have been discolored through the use in dipping operations involving copper and copper containing materials by the addition thereto of 0.1 to 1.0% of maleic acid.

JOSEPH L. KRIEGER. 

